Gas is becoming a more and more attractive fuel for ships' and other marine vessels' prime movers and auxiliary engines. Particularly but not exclusively natural gas (NG) is feasible due to its availability. Natural gas is a gaseous mixture in the ambient circumstances consisting primarily of methane and small amounts of ethane, propane, butane and nitrogen. It has a high hydrogen content relative to coal, so when combusted it provides inter alia low amount of emissions, very clean burning process and it is basically free of contaminants. Particularly in cruise vessels, ferries and so called ropax vessels, where passengers are on board, the absence of soot emissions and visible smoke in the exhaust gases of the ship's engines is a very important feature facilitated by using NG as fuel for the engines but also for vessels carrying goods and bulk materials. Usually natural gas is stored as liquefied natural gas (LNG) at temperature of about −162° C.
EP0069717 B1 discloses a method of utilising compressed boil-off gas from cryogenic liquids as fuel in a dual gas/oil burning diesel engine on board a vessel. In the method the compressed boil-off gas is introduced into the combustion chamber of the engine during combustion by high pressure injection which is controlled in accordance with the combustion pressure, desired revolutions per minute and output power. The system comprises a gas compressor whose suction side is connected to the storage tank containing the cryogenic liquid, and a buffer storage tank between the storage tank engine, which buffer storage tank receives compressed gas from the compressor. Thus the solution of EP0069717 B1 allows only the gas in gaseous phase in the storage tank to be utilized as fuel for the engine.
WO 2008000898 discloses fuel system for a gas-driven piston engine in a marine vessel, in which gas is stored in at least one fuel storage tank in the marine vessel as liquefied gas, and which fuel system further comprises a separate fuel feed tank in connection with the fuel storage tank. The fuel feed tank is a heat insulated pressure vessel in which the gas is stored in liquid phase and at elevated pressure. The gas is also in liquid phase in the fuel storage tank, in which prevails only hydrostatic pressure caused by the liquid gas. The publication shows also a heat exchanger arranged in a fuel feed line which connects the fuel feed tank to the gas driven piston engine, by means of which the liquefied gas may be evaporated prior to feeding to the piston engines. Since the pressure in the fuel storage tank is only hydrostatic pressure caused by the liquid gas there is a need for dedicated compressors to prevent excessive pressure build-up in the storage tanks by transferring the gas to the fuel feed tank at higher pressure. Due to the coupling of the storage tank and the feed tank, WO 2008000898 has a shortcoming particularly in the flexibility of utilizing the gas as fuel for the engine.
Although the fuel system shown in WO 2008000898 may be advantageous as such, there has recently come out demands to further develop such a fuel system. It is an object of the invention to provide a fuel feeding system for storing liquefied gas and feeding gaseous fuel to be used in a piston engine, which provides more flexible utilization of the gas as fuel for a gas-operated piston engine.